This invention relates to video scrambling signals that are transmitted or sent over cable and, in particular to a method and apparatus for overcoming the defeat of video signal scrambling processes caused by fine mistuning of a tuner circuit in a television set.
With the fine tuning feature present in many television sets and VCRs, many current television scrambling systems are vulnerable to this feature. By fine tuning (mistuning) in just the correct way, a viewable picture is possible from a scrambled video signal. Such scrambled video signals, for example, may come from sync suppression scrambling systems and/or from video signals where the horizontal sync pulses are position and/or edge modulated.
The fine tuning feature of a television timer shifts the tuner""s local oscillator to shift the video spectrum within the tuner""s intermediate frequency (IF) bandpass filter. See for example, FIGS. 1A, 2A and 3A-3E. If the fine tuning is set to be slightly off center (mistuned), the mixed down video in the IF bandpass (FIGS. 3C, 3E) will not be optimal and will exhibit some middle to high frequency emphasis at the tuner""s demodulator output (FIG. 4). As a result, the video signal will have leading and/or trailing edge emphasis or spikes in the time domain. The television""s (TV) sync separator will sense the over emphasized negative or trailing edges as xe2x80x9csyncxe2x80x9d signals. Generally, the end of line picture information has some of the greatest negative edge information and becomes a sync signal when fine mistuned. As a result, a viewable picture is achieved on the TV set even though the video signal used sync suppression scrambling. Under correctly tuned conditions, the TV set would show a concealed and unviewable picture. The use of frequency equalizing the scrambled video, for example via fine mistuning, not only works to defeat the scrambling feature of sync suppression systems, but also to defeat those scrambling systems with position or edge modulated horizontal syncs.
Therefore there is a need for a countermeasure to video frequency equalization via use for example of the television""s fine tuning control, such that various existing signal scrambling systems continue to provide concealed or unviewable signals.
The present method for maintaining concealment provided by sync suppression and sync modulation systems, under fine mistuned circumstances in a TV set, is to deemphasize the video frequency response in the scrambled signal prior to the modulator. At the descrambler end, a pre-emphasis in video frequency response is done to obtain an overall flat frequency response from transmitter to descrambled video output.
Yet another method that identifies that the end of line program content is the source for a viewable picture during fine tuning (mistuning), changes the fall time of the negative edge of video near the end of the television line.
Where sync modulation exists, there are other methods available to maintain concealment when the television set is fine mistuned to defeat this type of scrambling. One method that is prior art, is to have the trailing edge of video follow the sync modulation.
In this invention, there are several other ways to improve on the concealment. One way is to make the last 2 microseconds of the active video line close to a peak white level and then modulate the falling edge of this peak white level to follow the sync modulation. This peak white level guarantees that the trailing edge of the end of video line will be picked up by the fine tuning. A huge position modulated negative spike caused by fine tuning (mistuning) will be picked up as a sync signal and will maintain concealment via the position modulation.
Yet another way to maintain concealment of a sync modulated scrambling mode when fine mistuning is used to offset it, is to use a combination of peak white level and a waveform that follows the sync modulation during the last 2 microseconds of the end of the active video line. Upon addition of a waveform typically at about 500 Khz to 2 Mz, the waveform becomes enhanced during the fine tuning and thus appears as sync signals to the TV set. Since this waveform is also position modulated, it will cause the TV set to deliver a concealed picture. In short, under normal or correct fine tuning, the sync modulation causes the concealment of the video signal on a TV set. When the fine tuning is incorrect, the combination of the peak white level and/or waveform that is position modulated then causes the TV set to still deliver a concealed picture.
Still a further alternative embodiment includes various forms of vertical sync suppression and insertion, particularly in a sporadic manner. Thus, reinsertion of vertical sync pulses, although not necessarily standard vertical sync signals, along with the horizontal concealment techniques of previous mention provides enhanced signal concealment, particularly when using non-standard and/or sporadic vertical sync pulses in the VBI period. To this end, one technique of the invention removes some or all original vertical broad sync pulses and inserts a VBI with new vertical sync pulses of non-standard vertical sync locations, line fill signals and/or blanking or other signal voltages, in a sporadic or periodic manner.